Sintered ceramic article with porous region

ABSTRACT

A sintered metallic oxide article is disclosed comprising a non-porous body having at least one porous region formed therein in the general shape of a hollow cylinder, said region placing opposite sides of the body in flow communication. A method is set forth for producing the porous region with a preselected effective open cross-sectional area.

This invention is directed to useful articles of sintered metallic oxideand, in particular, to the provision of porous regions in otherwisenon-porous bodies. One such useful article described herein is thehousing for a reference electrode for high temperature aqueous systems.

DESCRIPTION OF THE INVENTION

A sintered metallic oxide article is disclosed comprising a non-porousbody having at least one porous region formed therein in the generalshape of a hollow cylinder, said region placing opposite sides of thebody in flow communication. A method is set forth for producing theporous region with a preselected effective open cross-sectional area.

BRIEF DESCRIPTION OF THE DRAWING

The features of this invention believed to be novel and unobvious overthe prior art are set forth with particularity in the appended claims.The invention itself, however, as to the organization, method ofoperation and objects and advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawing wherein:

FIG. 1 is a view in section (including an enlarged portion) through areference electrode employing a sintered metallic oxide housingconstructed according to one embodiment of this invention and

FIG. 2 is an enlarged view in section of the end of an electrode housingwith a tapered plug inserted according to the preferred process prior tofiring.

MANNER AND PROCESS OF MAKING AND USING THE INVENTION

The invention is described herein in connection with the construction ofa one-piece ceramic housing for a silver/silver chloride referenceelectrode suitable for use in measurements in high-temperature,high-pressure aqueous environments. Conventional reference electrodes,such as those described by Vermilyea and Indig [J. Electrochem. Soc.,Vol. 119, page 39 (1972)] employ a tetrafluoroethylene housing and,therefore, are susceptible to rapid hydrogen permeation. Tests of areference electrode configuration employing the one-piece housing ofFIG. 1 indicate that this construction operates well both in static andflowing systems at temperatures and pressures up to at least 290° C. and1500 psi in the presence of hydrogen.

In order to accommodate operation at such high temperatures, it isnecessary that provision be made for the escape of more than 25 percentof the electrolyte solution contained in the electrode housing at 25° C.before the system can equilibrate at about 290° C. This is accomplishedwith the microporous junction of preselected effective cross-sectionalarea according to this invention.

Referring to FIG. 1, the reference electrode 10 compriseslongitudinally-extending tubular housing 11 of sintered zirconia havingelectrode 12 suspended therein. Housing 11 is referred to as being"one-piece", because the tubular portion thereof is sintered, albeitimperfectly, to the close fitting plug 21. Electrode 12 is made of highpurity silver with the lower end thereof (shown darkened) coated withsilver chloride and immersed in a potassium chloride electrolytesolution 13. The upper end of ceramic housing 11 is provided withsealing member 14 made of polytetrafluoroethylene containing 40 percentby weight zirconia in order to reduce expansion and distortion at hightemperatures. Electrical isolation of electrode 12 is completed withceramic member 16, which biases member 14 into sealing engagement by theapplication of pressure thereto by threaded fitting 17. Threadedconnection 18 accommodates installation of the reference electrode 10through the wall of an enclosure containing an aqueous system in whichmeasurements (e.g pH) are to be made.

The use of a reference electrode employing a glass housing for themeasurement of pH is illustrated in U.S. Pat. No. 4,264,424--Niedrach.The pH sensing membrane of the Niedrach hydrogen ion sensor is made ofan oxygen ion conducting ceramic, such as stabilized zirconia. TheNiedrach patent is incorporated by reference. Various compositions ofmetallic oxide powder are described in U.S. Pat. No.3,429,962--Krystyniak. In the practice of this invention the powderswere consolidated into a body of the desired shape by isostatic pressingat 15,000 psi and the resulting body was then prefired at 1250° C. forone hour in air.

The prefired metallic oxide body is provided with a through hole to beclosed off, a prefired metallic oxide plug dimensioned to fit closely inthis hole is inserted in the hole and the resulting assembly is fired.The firing sinters the prefired metallic oxide members of this assemblyrendering them non-porous with the plug surrounded by and imperfectlysintered to the metallic oxide body.

The preferred ceramic composition for the fabrication of housing 11(which includes plug 21) employs 5 weight percent calcia as thestabilizing agent for the zirconia and 2 weight percent alumina isadded. The alumina enhances liquid phase sintering of the metallicoxides and eliminates connected porosity. Any alumina on the externalsurfaces of the sintered housing is leached out upon exposure to waterat 288° C. leaving a very stable, inert, non-porous and (at 288° C.)electrically insulating structure. In some instances the connectedporosity extends to the outer surface to a sufficient extent to enableuse of the housing without further modification. In those instances inwhich the flow communication characteristics of the annulus are suchthat there is insufficient porosity to accommodate the significantchanges in aqueous solution density at the operating temperature, whichrequire that solution be able to escape from within the electrolytechamber, this can be readily accommodated by sequentially cutting thinslices (e.g. each approximately 0.1 inch thick) from the end of theunified tubular portion of housing 11 and close-fitting plug 21imperfectly sintered thereto. By this expedient the connected porositybetween the inside and outside of the housing via the porous annulus canbe increased. A simple test to determine whether or not the extent ofporosity has been reached to the desired extent is to apply a gas (e.gnitrogen) under pressure of about 10 psi to the interior of the housingand observe the extent of bubble formation in a thin film of water onthe outer surface of the annulus.

An indication of the effective open cross-sectional area of the porousannulus can be determined by the use of electrical impedancemeasurements at constant temperature. Thus, such measurements have beenmade with housing 11 resting with its tip at the bottom of a stainlesssteel beaker filled with an aqueous solution containing 0.1 molar KCl tothe same level as an identical solution in the electrolyte chamberwithin housing 11. The value of electrical resistance between the beakerand a platinum wire touching plug 21 at its inner surface was used tocalculate the cross-sectional area and diameter of an equivalent roundhole the length of the plug. Examples of the results of such impedancemeasurements are shown in Table 1. Among the symbols employed: ρ is theresistivity (ohm cm) and κ is the conductivity of the KCl.

                  TABLE 1                                                         ______________________________________                                        IMPEDANCE MEASUREMENTS ON ZIRCONIA                                            REFERENCE ELECTRODE HOUSINGS                                                  Plug                                                                          Length (mm)                                                                              Resistance (kΩ).sup.(1)                                                              A (cm.sup.2).sup.(2)                                                                     d (μm).sup.(3)                          ______________________________________                                        1   4.0 ± .05                                                                              77.0 ± 0.05                                                                            4.25 × 10.sup.-4                                                                 233                                      2   2.5        200 ± 2   1.04 × 10.sup.-4                                                                 115                                      3   1.8        270 ± 10  0.54 × 10.sup.-4                                                                  83                                      4   2.0        250 ± 30  0.66 × 10.sup.-4                                                                  92                                      ______________________________________                                         .sup.(1) 0.1 m KCl, 1000 Hz, 21.8° C., 1.21 × 10.sup.-2          ohm.sup.-1 cm.sup.-1                                                          .sup.(2) R = ρ1/A, A = 1/Rk for a uniform cross section A, length 1       .sup.(3) d = (4A/ π).sup.1/2, the diameter of a round cylinder with        cross section A.                                                         

The accuracy of the measurements made in this way depends upon completefilling of the continuous porosity in the annular region.Characterization by gas flow is simpler and quicker.

FIG. 2 shows a preferred embodiment of this invention. Instead ofemploying a prefired metallic oxide plug machined in the form of a rightcircular cylinder, plug 25 is provided with a slight taper (greatlyaccentuated in the drawing). In the same manner as describedhereinabove, the plug is fitted into the uniform bore of tube 11 beforethe ceramic is fired to final dimensions. The fired assembly possesses aporous annulus having a substantially uniform gradation of porosity.Thus, successive thin slices taken from the tip of tube 11 expose moreand more imperfectly sintered regions in the annulus assuringselectivity of the desired porosity in the finished product.

Although this invention has been illustrated by describing the manner ofpreparation of a porous region in the shape of a hollow cylinder, theinvention is equally applicable to the preparation of other geometricshapes providing preselected porosity through sintered otherwisenon-porous metallic oxide walls or bodies. An example of a use for asintered metallic oxide enclosure provided with a plurality of suchporous regions would be as a sparger for the introduction of a gas intosurrounding liquid. The invention, of course, is not limited to anyparticular metallic oxide or mixtures of metallic oxide.

What we claim as new and desire to secure by Letters Patent of theUnited States is:
 1. A sintered metallic oxide body defined at least inpart by two opposed surfaces and consisting of a plurality of non-poroussintered metallic oxide regions and at least one imperfectly sinteredmetallic oxide porous region therein; each porous region being in thegeneral shape of a hollow cylinder defining one of said non-porousregions enclosed therein and having a second non-porous regioncontiguous with its outer boundary, said porous region having apreselected effective open cross-sectional area and extending into saidbody a distance sufficient to place said opposed surfaces of said bodyin flow communication through said porous region.
 2. The metallic oxidebody of claim 1 in which said body is in the shape of alongitudinally-extending tube closed at one end and the porous region islocated in said closed end.
 3. The metallic oxide article of claim 2 inwhich the metallic oxide of all metallic oxide regions is stabilizedzirconium oxide.
 4. The sintered metallic oxide body of claim 1, whereinthe porous region has a substantially uniform gradation of porosity. 5.A reference electrode construction comprising a one-piece sinteredmetallic oxide housing enclosing a sealed electrolyte reservoircontaining liquid electrolyte and an electrode spaced from said housing,the liquid electrolyte contained in said reservoir being in flowcommunication with the exterior of said housing solely through animperfectly sintered porous metallic oxide region having a preselectedeffective open cross-sectional area, said porous region being in thegeneral shape of a hollow cylinder extending through the otherwisenon-porous sintered metallic oxide wall of said housing.
 6. Thereference electrode construction of claim 5 wherein the housing is alongitudinally-extending tubular member and the porous region is locatedin an otherwise closed end thereof.
 7. The reference electrodeconstruction of claim 6 wherein the electrode construction in contactwith the electrolyte is silver/silver chloride and the liquidelectrolyte comprises potassium chloride solution.
 8. The referenceelectrode construction of claim 5 wherein the porous region has asubstantially uniform gradation of porosity.